Chloroborane complexes, the precursors to organohaloboranes, have been prepared by a variety of methods. See H. C. Brown, S. U. Kulkarni, J. Organomet. Chem. 1982, 239, 23-41. For example, lithium or sodium borohydride and boron trichloride give chloroborane-ether adduct according to the following reaction: ##STR1## where M=Na, Li. See H. C. Brown, P. A. Tierney, J. Am. Chem. Soc. 1958, 80, 1552; H. Noth, H. Beyer, Ber. 1960, 93, 225; U.S. Pat. No. 3,026,329.
A mixture of borane-tetrahydrofuran adduct and boron trichloride form chloroborane-tetrahydrofuran adduct according to the following reaction: ##STR2## See D. J. Pasto and P. Balasubramaniyan, J. Am. Chem. Soc. 1967, 89, 295; D. J. Pasto and S. Kang, J. Am. Chem. Soc. 1968, 90, 3797.
Borane-dimethylsulfide adduct (DMSB) and dimethylsulfide boron trichloride give dimethylsulfide chloroborane according to the following reaction: EQU 2Me.sub.2 S.multidot.BH.sub.3 +Me.sub.2 S.multidot.BCl.sub.3 .fwdarw.H.sub.2 BCl.Me.sub.2 S
See K. Kinberger and W. Siebert, Z. Naturforsch., B 1975, 30, 55; H. C. Brown and D. Ravindran, J. Org. Chem. 1977, 42, 2533; H. C. Brown, Inorg. Chem. 1977, 16, 2938. These chloroborane ether and sulfide adducts have been used in hydroboration reactions as described in detail below.
Finally, gas phase reaction of diborane and boron trichloride gives dichloroborane according to the following reactions: EQU B.sub.2 H.sub.6 +4BCl.sub.3 .fwdarw.6BCl.sub.2 EQU 2BCl.sub.3 +5B.sub.2 H.sub.6 .fwdarw.6B.sub.2 H.sub.5 Cl
See J. Cueilleron, J. Bonix, Bull. Soc. Chim., France 1967, 2945. Although these gas phase reactions were not conducted for subsequent hydroboration reactions, they demonstrate that chloroboranes can be prepared in the absence of coordinating ether or sulfide ligands.
Diisopinocampheylchloroborane (hereafter referred to as DPC) is a very useful chiral reducing agent for prochiral ketones and imines. DPC can be prepared from .alpha.-pinene and chloroborane diethyl etherate. H. C.Brown, P. K. Jadhav. J. Am. Chem. Soc. 1983, 105, 2092. A variation of this process using dimethyl sulfide chloroborane has been elaborated by G. Bir and A. O. King. G. Bir, D. Kaufmann, Tetr. Letters 1987, 28, 777; and A. O. King et al., J. Org. Chem. 1993, 58, 3731-3735.
Brown purports to obtain high optical purity DPC through a two-step procedure. See U.S. Pat. Nos. 4,772,752 and 5,043,479. In this procedure, diisopinocampheylborane, a moisture and thermally sensitive intermediate is isolated from the reaction of .alpha.-pinene and DMSB. Subsequent addition of hydrogen chloride gives the final product, DPC, as a moisture sensitive solid, which is isolated from the solution. On an industrial scale this complicated process presents several technical difficulties including the handling of thermally-sensitive solids as well as the handling of moisture sensitive product.
A. S. Thompson, in J. Org. Chem. 1992, 57, 7044-7052, and A. O. King, supra, have shown that solutions of DPC made from &gt;70% ee .alpha.-pinene and DMSB or dimethyl sulfide chloroborane can be used in excess to achieve high optical purity reduction products, without the need for the handling of the moisture sensitive DPC solid.
All of these processes for making DPC involve use of chloroborane-diethyl ether adduct, which is thermally unstable, or dimethyl sulfide chloroborane adduct, which is notoriously malodorous. Some of the ether and sulfide ligands cause problems in the formation and use of the organochloroboranes. For example, diorganochloroborane formed from a haloborane-tetrahydrofuran adduct can cleave tetrahydrofuran rendering the borane compound unusable. Furthermore, the sulfide adducts leave a sulfur smell on the final product. Development of a method for hydroboration of .alpha.-pinene by chloroborane to form DPC without the use of ether chloroborane adduct or sulfide chloroborane adduct is, therefore, very desirable.